How do you get the supercannon for Age of Empires 3?

It's called a Great Bombard and only the Ottoman civilization can get them.

Play a skirmish as the ottomans and they should be available from the home city in the industrial (4) age. You can also get the factory card, use it to build a factory, and assign it to build heavy artillery. Remember that these cannons take up HUGE population.

Recap:

Only Ottomans can get them

They can't be built except from a factory/home city.

You can upgrade them to Imperial Bombards for 1500 Wood and Gold =]

Take up HUGE population (like 8 per Cannon)

Hope this helps=]